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Vol. 20, No. 12
CHEMISTRY AND THE FOOD INDUSTRIES Symposium presented before the Division of Agricultural and Food Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928
Chemistry and the Canning Industry W. D. Bigelowl NATIONAL CANNSRS ASSOCIATION, WASHINGTON, D. C.
HEMISTRY and bacteriology go hand in hand in the service they perform for the canning industry. They are used together in the examination of samples to determine the cause of spoilage or to ascertain the reason for any abnormality in appearance or flavor. For the purpose of the present discussion, examples have been chosen from the series of studies carried on during the past ten years, which have added much to our knowledge of the fundamental principles of processing. Canned foods are simply cooked foods. They are essentially the same in composition and nutritive value as foods freshly cooked from the raw product in the kitchen."#* In the study of their composition and nutritive value, canned foods do not offer unique problems. The methods used with foods prepared by other methods are, of course, available. Some years ago it was assumed that the vitamin content of foods was destroyed or a t least greatly reduced by the canning operation and perhaps by subsequent storage. This has been Extensive studies have been found to be untr~e.6~,63,64,6~,'6 made of the influence of canning on vitamins A, B, and C. It has been found that vitamins A and B are not affected appreciably by the canning operation and that vitamin C is generally destroyed to a less extent in canning than in cooking the raw product in an open receptacle, as is ordinarily done in the kitchen. This is because vitamin C is destroyed by oxygen rather than by heat and canned foods are processed (sterilized) after the can is sealed. Even when the canned product is heated for the table it contains no oxygen in solution and the amount of vitamin C destroyed is inappreciable. After the product is canned the vitamin content appears to remain unchanged during storage, and there is not the diminution in vitamin C which occurs in raw products during storage through the action of atmospheric oxygen. I n the canning of foods, sugar or salt or both sugar and salt are frequently added for flavoring, and water is used either to secure the desired consistency or to fill interstices within the can. The water, sugar, and salt used require the attention of the chemist for somewhat different reasons than apply to other food industries. Sugar must be practically free from sulfur dioxide. Even small amounts of that substance tend to accelerate the corrosion of the inside of the can with certain fruits. Also, in the processing of fruits, any sulfur dioxide that may be present is reduced to hydrogen sulfide and forms a tarnish of tin sulfide on the inner surface of the can, sometimes discoloring the product. Such a tarnish is normal with protein-bearing vegetables, such as peas and corn, but is abnormal with fruits and arouses the suspicion of the consumer. It has also been pointed out very recently that even refined granulated sugar sometimes carries spoilage bacteria.24 This will be discussed later.
C
1
Director of Research Laboratories, National Canners Association. in text refer to the selected references at the end of the
* Numbers article.
I n canning some products. soft water is preferable to hard. An excessive amount of calcium in the water has a toughening effecton peas and shelled beans.5l With some other products, for example, beets, calcium in the water may combine on the surface of the product with oxalic acid naturally present, and form a white coating, which is undesirable. The composition of the water is also of interest fromother standpoints. When cans of food are submerged in hard water during the sterilizing process, an etching or incrustation is sometimes found on the outside of the cans. This a t least dulls the luster of the cans, and sometimes occasions rusting. Water containing an abnormal amount of sodium bicarbonate also etches the outside of the cans. Only highly purified salt should be used. An excessive amount of calcium in the salt is objectionable for the same reasons as in water. Examination of Canned Foods
Chemical and bacteriological methods are often important for the examination of canned foods, especially for the purpose of detecting the cause of any abnormality in the product. An abnormal appearance or flavor may be due to the nature of the raw product or to the details of the canning operation. I n the examination of canned foods, attention is first given to the can. The analyst must understand the principles of can making and be thoroughly familiar with the proper construction of the can. Only such knowledge and experience can enable him to determine whether the can is tight. It sometimes happens that the seal is sufficiently tight to exclude bacteria but loose enough to admit air. In such a case, oxygen accelerates corrosion9 and may lead to the perforation or swelling of the can, to various forms of discoloration, or to a staleness or other abnormality of flavor. The appearance of the inside of a can often suggests the nature of the examination that should be made. Black discoloration of the contents due to several causes has been explained, for instance, in a great variety of products, such as corn,64,69 peas, shrimp, hominy,eg beets,so and sweet potatoes.70 This is usually due to sulfide of iron or copper, or to a combination of iron with polyphenol. A yellowish or brownish color sometimes results from excessive sterilization or too high a temperature of storage. Swelling of a can may be due to bacterial decomposition, to hydrogen resulting from the action of the contents on the tin plate of the can, or, in sirups, to a form of chemical decomposition known as frothy fermentation (Schaumgiirung). Processing Studies
During the last dozen years much has been added to our knowledge of the fundamental principles of processing, and this knowledge has been applied in a practical way to the manufacture of canned foods. Much information has been
December. 1928
INDUSTRIAL A N D E.VGINE%ERING CIIEXISTRY
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accumulated on the hca.t penetration of canned food~~7.~8~2o hut the study given to the subject has added much to our and study has also been made of the heat resistance of those knowledge. bacteria which are hardest t o kill in processing and which arc It was formerly assumed from the relative position of tiii ordinarily known in the industry as spoilage bacteria.2z~27~28~*~ and irori in the electrochemical series that iron was anodic Attention has also heen given to the hydrogen-ion coiiceii- in the tin-iron couple of tin plate. This, it was assumed, w a ~ trat,ion of foods, both before and after c a r i n i ~ i gsiid , ~ ~ the pro- the fundamental eanse of perforations in canned fruits. found influence i t has on the process necessary for sterilizing Contrary to our expect,ation, it has been shown that with various kinds of foods. The pII value of fruits, for instance, mimed fruits the tin is anodic to t,lie iroii of t,he tin an.'^,'^ is ususlly between 3 and 4 arid that of tomatoes shout 4.3. Data noil~nvailablclead to the coliclusioil that if this m r e not This acidity is too great, to SUppWt the life of spore-bearing the c&setlie tiii can would not be a siiitahle eontaineu for iiiost hactcria, and, therefore, it, is only necessary in these prodrrcts of our canned fruits. to destroy t.lie vegetative fisrn. Vegetables have a higher pH In order to have a clear uiiderstandiiig of the act,ion of value, and iii procerping their1 it is necessary to kill tlie spores. t,he various fruits 011 tile c.an, it should he noted that colored fruits d i o s e colors :we anthocyan pigiiieiits, that is, clierries m d berries, are p ~ l i e din eriairicled cans in order to prevent hlcaclring. It hns been pointed ont that the anthocyan pigments are effective &~i~~lnsisers.'~ Fruits in plain cans give 110 serious ilifiiculties a,s a result of corrosion. What corrosiori there is is distrihu1,cd fairly nrriformly over thc entire iinier surface of tlie c ~ n . I n eo:meled cans, tlie eorrosioii diicli rwilts is largely contiiied to i.elati.rely srnall d i e r e t,he p h t e has 11ecn snbjected to the grc:rtest in forming the cnii;" t,liiit.is, in tlie d o n l k settin at tlie elid of tlie cini atid along tlie sidr scan. Iii a plairi c : tlie ~ Inrge anodic iaroa of tiii tdfonli; rffectivo protect.ion for tlie iron, while tlie small iirea of eiit,lrodic iroii has l i t t l e effect up:in tiii c,orrosion. In m i ciinineled can, nlirc area of tlie esposed iroii has been enord hccaiise of the iiuiiierous iircaks in both tlre coatings at points n h r r e tlie metal has heen nrhjeoted to str;tiii. hlt~iiougli t,hc froit x i & Imve little ContinUoU8 Cookers W i t h Lacquering Baths in Foreground effect i n curroiling tiii, the conibiiirxl action d bile neids and Some of these spores are very resistant to heat,.'g~2?~?2 These the fruit cdvrs wliich act as depolarizers h i ~ sa considerable resistant spore-heariiig lmeteria have been classified and their corrosive effrrt o r 1 tile tin. Aioreoi-er, the relnt.ively neater lieat resist:ince has heen ilot,ermined. Considershlc work Itas area of cathodic iron of a11 eiiaincled c:ui like corrosive actio,, 011 tlie t.in :mode. The resnlt is lliat witliin nlso been done in bneteriologieal fie111 surrcys iri urliich the sources of these spoilage hactcrin hare bceii stitdicxl.~~Sugi~r relatively short periods there may he iinm iron thnn ti11 eshas been fonird to be the chief soiirci:. Even tile rcfiiied graiiin tlre industry r n ~ yc:irry the spores of twin. T h e y :ire present iri s~n:dIiinrnhers and during t,lit: last. two years a n ext,ensive st,iidy has bee11 iiiade of the eoiditioris i n canning plants diicli m:iy lead to the multipliciLtion of tliesc buctcri;l, t,o such ~ i extent i that t.liry h(xotrie a f:ictut iii pro irig. In the hncteridogical field surx'ey nicntioned ahove has heeri possiblr to dcteriiiiiie wlietller tlict c:iinieriei visited afforded conditions that rr-oiild permit, this rnultiplicatioir of t.lie spoilage hncteria. Where .;och conditions were foririd remcclies were reiidily suggested. Xore inrportrtrrt t11;irr this, the field survey established certain principles which r n w t be ohserved to prcr e n t sncli inultiplication of hscteri:t. For iiistancc, voodeii tanks when used for sirups and lxiors arc somrtimes siifKciently porous to harbor bacteria witliiu the wood. This ohserration hiis led to the suggest,ioii that, hnril sixfared tanks lie used in phicx of those ninde of \~ood. Tin Plate
innjor suhjecls wit,li Ir-hich cliernists and sllied induiirirs have heel, coniirs. When the work I v a s hoped t h n t a plate might be nianihctiircd i h i c h \voirld vitlistthiid the nct.ion of c:~mmJEnds (if nil kinds for a snffirient t,iinr to satisfy the dein:ti~lsof eonimcrcc. It. i m s hoped t.li:rt t i wighi, of tin coating might he fiiiiiirl whicli woulil protcct tlie stccl base so well, or t1i:it t l i e steel bnsc itself rniglit he ~ n i i d cof siiuh a cornpositio~r or iii siidi a iiimiicr, thnt the serviw due tin plntc i n p:~.r:k~ra' can.; might he greatly incn::iwd.'j.'E Tlris has riot I J Cfoniitl C ~to ~be the mse,
Canning Pineapples in Hawaii
posed i n : ~ i iciio,nieled The inni, having thus lost the proteri.ire c i h t of anodic tin, is able to set uplocal couples wliieli rcsnlt i n hydrogen foniiatioii and eveiituiilly perforations. I-:nkiis factors co~itrihntetow:vd reiidering t,hc tin anodic to iron in c n i n r r d Emits.'* The tin as it corrodes is to w. large extent agnin rc-l,recipitated hy the fruit constituents. In herries tlie til, is largely ndsorhcd by the s ~ e d s . The ~ pulp tlie fruit likcwise tends t,o precipitate the tin in t,he ahseiice
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INDUSTRIAL AND ENGINEERING CHEMISTRY
of seeds. In this manner the concentration of stannous ions is maintained a t a relatively low figure and the potential of tin is materially affected.2J2J8 It has been shown that the single potential of tin is affected by hydrogen-ion concentration to the extent that, although tin may be cathodic to iron in very weak acid solutions, a t a higher hydrogen-ion concentration the conditions are reversed and the tin becomes anodic. In accordance with this, it is the general experience in the industry that difficulties from corrosion are frequently the greatest with the less acid fruits.I2 Tin has an inhibiting effect on the iron corrosion in addition to its anodic relationship. Tin in solution, even though in extremely small concentrations, such as a few parts per million, has a very marked effect in inhibiting iron corrosion. It has been shown that such small amounts of tin enormously increased the overvoltage on iron, and it is believed that the protective action of soluble tin is due to this effect.I2 As already stated, the most serious corrosion takes place in the double seam a t the junction of the body and end of the can. At this point there is a relatively deep and narrow channel. Diffusion into the bottom of this channel is naturally limited. Since the available depolarizing agents, such as the anthocyan pigments, are proportionately less than the available acid, the pigments in the bottom of this channel are exhausted first. This permits of the formation of a concentration cell, with the bottom of the channel as anode and the top of the channel as a cathode, which is continually supplied by depolarizing agents present in the fruits. This mechanism of the corrosion, already described,’ is probably a material factor in augmenting corrosion a t the seam. Selected References TINPLATEAND CORROSION 1-Baker, “Disappearance of Oxygen in Canned Food Containers,” 8 f h Intern. Cong. A p p l . Chem., 18, 45 (1912). 2-Bigelow and Bacon, “Tin Salts in Canned Foods of Low Acid Content, with Special Reference t o Canned Shrimp,” U. S. Dept. Agr., Bur. Chem., Circ. 79 (1911). 3-Bigelow, “Tin in Canned Foods,” J. IND. E N O .CHEM.,8, 813 (1916). 4-Clough, Shostrum, and Clark, “Lime Sulfur on Canned Gooseberries,” Canning Age, 5, 531 (1924). 5-Culpepper and Moon, “The Effect of Nitrates upon Corrosion in Tin Cans,” Ibid., 9, 619 (1928). 6-Dill and Clark, “Can Corrosion and Blackening in Certain Marine Products,” IND.EXG.CHEM.,18, 560 (1926). 7-Evans. “The Newer Electrochemical View of the Corrosion of Metal,” J . Soc. Chem. Ind., 43, 222 (1924). &Goss, ”Adsorption of Tin b y Proteins, and I t s Relation to the Solution of Tin by Canned Foods,” J. IXD.ENG.CHBJI., 9, 144 (1917). Q-Kohman, “Oxygen and Perforations in Canned Fruits,” Ibid., 16, 527 (1923). 10-Kohman and Sanborn, “The Nature of Corrosion in Canned Fruits,” Zbid., 16, 290 (1924). 1928, p. 11-Kohman, “Research Findings.” The Canner, Convention KO., 144; Canning Age, Convention Digest, 1928, p. 227. 12-Kohman and Sanborn, “Factors Affecting the Relative Potentials of END.CHEM.,20, 1373 (1928). Tin and Iron,” IND. 13-Kohman and Sanborn, “Tin Plate and the Electrochemical Series,” Zbid., 20, 76 (1928). l C L u e c k and Blair (with reply by Kohman and Sanborn), “The Electrochemical Relations of Iron and Tin,” 54th Meeting of American Electrochemical Society, Preprint 22 (1928). 15-Technical Committee, “Relative Value of Different Weights of Tin Coating on Canned Food Containers,” Natl. Canners Assocn., 1917. l6--Technical Committee, “Canned Food Containers: A Study with Special Reference t o the Influence of the Steel Base on Resistance t o Perforations,” Natl. Canners Assocn., Bull. 22-L (1923). BACTERIOLOGY AND HEATPENETRATIOX 17-Ball, “Thermal Process Time for Canned Food,” Natl. Research Council, Bull. 87 (1923). 18-Ball, ”Theory and Practice in Processing,” Canner, 27 (January 22, 1927).
Vol. 20, No. 12
19-Barlow, “A Spoilage of Canned Corn Due t o a Thermophilic Bacterium,” Thesis for M.S. degree, University of Illinois, 1912. 20-Bigelow, Bohart, Richardson, and Ball, “Heat Penetration in Processing Canned Foods,” Natl. Canners Assocn., Bull. 16-L (1920). 21-Bigelow, “The Logarithmic Nature of Thermal Death Time Curves,” J . Infectious Diseases, 19, 528 (1921). 22-Bigelow and Esty, “The Thermal Death Point in Relation to Time of Typical Thermophilic Organisms,” Zbid., 27, 602 (1920). 23-Cameron and Esty, “The Examination of Spoiled Canned Foods. 2Classification of Flat, Sour Spoilage Organisms from Nonacid Foods,” Ibid., 39, 89 (1926). 24-Cameron. Williams, and Thompson, “Bacteriological Field Studies in Canning. Thermophilic Contamination in the Canning of Peas and Corn,” Natl. Canners Assocn., Bull. 25-L (1928). 25--Cheyney, “A Study of the Micro-Organisms Found in Merchantable Canned Foods,” J . Med. Research, 40, 177 (1919). 26-Clark, Clough, Fellers, and Shostrum, “Examination of Canned Salmon,” S a t . Canners Assocn., Northwest Branch, 1923. “A Highly Resistant Thermophilic Organism,” J . Bact., 6, 27-Donk, 373 (1920). 28-Esty and Williams, “Heat Resistance Studies. I-A New Method for the Determination of Heat Resistance of Bacterial Spores,” J . Infectious Diseases, 34, 516 (1924). 29-Esty and Stevenson, “The Examination of Spoiled Canned Foods. I-Methods and Diagnosis,” Ibid., 36,486 (1925). 3@-Feelrs, “A Bacteriological Study of Normal Canned Salmon,” J . Bacf., 12, 181 (1926). 3l-Fellers, “Gaseous Spoilage in Canned Marine Products,” University of Washington, Fisheries Pub., 1, 229 (1927). 32-Hunter, “Bacterial Decomposition of Salmon,” J . Bact., 5, 353 (1920). 33-Hunter, “The Sources and Characteristics of the Bacteria in Decomposing Salmon,” Ibid., 7 , 85 (1922). 34-Le Fevre, “The Commercial Production of Sauerkraut,” U. S. Dept. Agr., Circ. 36 (1928). 35-Meyer, “Botulismus, Handbuch der pathogenen Mikroorganismen” (3rd edition) by Kolle and Wasserman, 1928. (This article contains a full bibliography on botulism.) 36-Morrison and Tanner, “Studies on Thermophilic Bacteria. IAerobic Thermophilic Bacteria from Water,” J . B a d . , 7, 343 (1922). 37-Morrison and Tanner, “Studies on Thermophilic Bacteria,” Botan GQZ., 77, 171 (1024). 38-Mickle and Breed, “ A Gaseous Fermentation of Tomato Pulp and Related Products,” N. Y. State Agr. Expt. Sta., Tech. Bull. 110 (1925). 39--0bst, “A Bacteriological Study of Sardines,” J . Znfeclious Diseoscs, 24, 158 (1919). 4@-Parmele, Fred, Peterson, McConkie, and Vaughn, “Relation of Temperature to Rate and Type of Fermentation and t o Quality of Commercial Sauerkraut,” J . Agr. Research, 36, 1021 (1927). 41-Prescott and Underwood, “Micro-Organisms and Ster in the Canning Industry,” Technology Quarferly, 10, 183 (1897); 11, 6 (1898). 42--Russell, “Gaseous Fermentation in the Canning Industry,” Wis. Agr. Expt. Sta., 12th A n n . Rept. 227, 231 (1895). 43-Savage, Hunwicke, and Calder, “The Bacteriology of Canned Meat and Fish,” Food Investigation Board, Special Reel. 11 (1922). 44-Savage and Hunwicke, “Studies in Sweetened and Unsweetened (Evaporated) Condensed Milk,” Ibid., 13 (1923). 45-Savage and Hunwicke, “Report upon Canned Fruit,” Ibid., 16 (1923). 46-Weinzir1, “The Bacteriology of Canned Foods.” J . Med. Research. 39, 349 (1919). 47-Werkman and Weaver, “Studies in the Bacteriology of Sulfur Stinker Spoilage of Canned Sweet Corn,” Iowa State College, J . Sci., 2, 57 (1927). GENERAL REFERENCES 48--Bigelow, “Storing Canned Foods in the Open Cans,” A m . Food J.. 13, No. 2 (1917). 49-Bigelow, “Swells and Springers,” Natl. Canners Assocn., Circ. 6-L (1923). 5>Bigelow and Stevenson, “Tomato Products,” Ibid., Bull. 21-L (1923). 51-Bigelow and Stevenson, “The Effect of Hard Water in Canning Vegetables,” Ibid., Bull. 20-L (1923). 52-Bigelow and Cathcart, “Relation of Processing to the Acidity of Canned Foods,” Ibid., Bull. 17-L (1921). 53-Bigelow, “The Inspection of Canned Foods,” J. IND.ENG.CHEM.,8 , 1005 (1916). 54-Bigelow and Miller, “A Cause of Dark Color in Canned Corn,” Natl. Canners Assocn., Bull. 6 (1915). 55-Bohart, “Special Enamel for Corn Cans,” Ibid., Circ. 10-L (1924). 56-Clark and Clough, “Crystals Found in Canned Salmon Are Not Glass,” PaciJlc Fisherman, 23, No. 10, 11 (1925). 57-Clark, Clough, and Shostrum, “The Function of Vacuum in Canned Salmon,” Ibid., 21, No. 5, 8; No. 6, 12; No. 7 . 13 (1923). 58-Clough, Shostrum, and Clark, “A Study of the Gases in Canned Foods.” Canner, 19 (November 7 , 1925).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
59-Fitzgerald, Bohart, and Kohman, “Black Discoloration i n Canned Corn,” Natl. Canners Assocn., Bull. 18-L (1922). fi-eidel, “Control of Corn Quality b y Chemical and Physical Analysis,” Canner, Convention h-o., 1925, p. 134; Canning Trade, Convention KO., 1925, p. 90. 61-Givens and McClugage, “The Effect of Heat and Age upon the Antiscorbutic Vitamin in Tomatoes,” Pvoc. SOC.Exptl. R i d . M e d . , 18, 164 (1921). 62--Gowen, “Effect on Quality of Holding Shelled Peas,” Canning Trade. Convention No., 1928, p. 7 4 ; Canning A g e , Convention N o . , 1928, p. 202; Canner, Convention No., 1928, p , 112. 03-Hess and Unger, “The Effect of Age, Heat, and Reaction on Antiscorbutic Foods,” J . Bid. Chem., 38,293 (1919). fi4--Kohman, “Vitamins in Canned Foods,” Natl. Canners Assocn., Bull. 19-L (revised 1927). B b K o h m a n , Eddy, et a!., “Vitamins in Canned Foods,” IND. EXG CHEM.,15,273 (1923); 16, 52, 1261 (1924); 17,69 (1925); 18,85, 302 (1926); 2 0 , 202 (1928). 66-Kohman, “Canned Foods,” Hygeio, 6, 639 (1928). 67-Kohman and Sanborn, “Storage Temperatures for Canned Fruits,” Natl. Canners’ Assocn., Bull. 23-L (1927). B&Kohman, “Phenol-Chlorine Water Pollution,” IND. ENG. CHEM., 111, 518 (1923). R+Kohman, “Lye Hominy: I t s Discoloration and a New Process for I t s Manufacture,” I b i d . , 14, 415 (1922).
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7@-Kohman, “Discoloration in Canned Sweet Potatoes,” Ibid., 13, 634 (1921). 71-Magoon and Culpepper, “A Study of the Factors Affecting Temperature Changes in the Container during the Canning of Fruits and Vegetables,” U S. Dept. Agr., Bull. 966 (1921). 72-Magoon and Culpepper, “Relation of Initial Temperature t o Pressure, Vacuum and Temperature Changes in the Container during Canning Operations,” Ibid., 1022 (1922). 73-Meister, “Controlling Consistency of Canned Corn,” Canner, 23 (January 23, 1926). 74-Meister, “Variations in Consistency of Canned Corn,” Ibid., 19 (June 18, 1927). 75--Meister, “Study of Solubility of Certain Metals and Their Effect on Color of Canned Corn,” Ibid.. 17 (May 12, 1928). 76-Miller, “Vitamins A, B, and C in Fresh and Canned Pineapple,” J . Home Econ., 16, 18, 74 (1924); 17,377 (1925). 77-Purcell and Hickey, “Occurrence of Struvite in Canned Shrimp.” A n a l y s t , 47, 16 (1922). 78-Savage, “The Methods Used for the Inspection of Canned Foods,” Food Investigation Board, Specin! R e p t . 3 (1920); 10 (1922). 79-Shostrum, Clough, and Clark, “A Chemical Study of Canned Salmon,” IXD.ENG.CHEM.,16,283 (1924). S+Stevenson, “Discoloration in Canned Foods,” Canning Trade, 22 (February 2, 1925).
Chemistry and the Beverage Industry F. M. Boyles JACK
BEVERAGES, INC., BROOKLYN, N. Y.
HE subject matter of this paper will be confined to
T
carbonated beverages, as distinguished from the socalled cereal beverages or brewed drinks. The beverage industry, strictly speaking, is not a chemical industry, but its progress and development are very closely tied up with our science. Although the chemist in this industry has not been as spectacular a performer as in many other industries, he has been responsible not only for the industry itself but very largely for what progress it has made. Of course we could not have attained the position we occupy today without our many truly wonderful mechanical appliances, credit for which is due to the mechanical engineer. It mas Priestley, the preacher-chemist, the discoverer of oxygen, who did the pioneer work with carbon dioxide gas, or “fixed air,” as he called it, and especially the solution of this gas in water, upon which the industry has been built. I could not describe to you nearly as interestingly as Priestley himself has done those first epochal experiments, which he records in this language: It was in consequence of living for some time in the neighborhood of a public brewery, a little after mid-summer in 1767, that I was induced t o make experiments on fixed air, of which there is always a large body ready formed on the surface of the fermenting liquor. A person who is quite a stranger to the properties of this kind of air would be agreeably amused with extinguishing lighted candles, or chips of wood in it, as it lies upon the surface of the fermenting liquor. Considering the near affinity between water and fixed air, I concluded that if a quantity of water was placed near the yeast of the fermenting liquor, it would not fail to imbibe that air, and thereby acquire the principal properties of Pyrmont and some other medicinal mineral waters. Accordingly, I found that when the surface of the water was considerable, it always acquired the pleasant aciduloiis taste that Pyrmont water has. The readiest way of impregnating water with this virtue in these circumstances is to take two vessels and to keep pouring the water from one into the other, when they are both of them held as near the yeast as possible; for by this means a quantity of surface is exposed t o the air, and the surface is also continually changing. In this manner, I have sometimes, in the space of two or three minutes, made a glass of exceedingly pleasant sparkling water, which could hardly be distinguished from very good Pyrmont, or rather Seltzer water.
One would naturally think that having actually impregnated common water with fixed air, produced in a brewery, I should immediately have set about doing the same thing with air let loose from chalk, etc., by some of the stronger acids. But, easy as the practice proved to be, no method of doing i t a t that time occurred to me. I still continued to make my Pyrmont water in the manner above mentioned till I left that situation, which was about the end of the summer of 1768; and from that time, being engaged in other similar pursuits, I made no more of the Pyrmont water till the spring of the year 1772. If water be only in contact with fixed air, it will begin to imbibe it, but the mixture is greatly accelerated by agitation, which is continually bringing fresh particles of air and water into contact All that is necessary, therefore, to make this process expeditious and effectual, is first to procure a sufficient quantity of this fixed air, and then to contrive a method by which the air and water may be strongly agitated in the same vessel, without any danger of admitting the common air t o them; and this is easily done by first filling any vessel with water, and introducing the fixed air to it, while it stands inverted in another vessel of water. The pressure of the atmosphere assists very considerably in keeping fixed air confined in water, for in an exhausted receiver, Pyrmont water will absolutely boil by the copious discharge of its air. This is also the reason why beer and ale froth so much in vacuo. I do not doubt, therefore, that by the help of a condensing engine, water might be much more highly impregnated with the virtues of the Pyrmont spring, and it would not be difficult to contrive a method of doing it.
Prophetic words, clearly pointing the way to the carbonated beverage industry! It is of interest to note here that Priestley’s source of carbon dioxide, namely, fermenting vats, is today the source of a very large part of the gas used in carbonated beverages. Priestley’s gas, however, with its entrained vapors, carrying along the flavors and odors of fermentation, could not be used in delicately flavored drinks. The chemist therefore has purified this gas by eliminating all of the objectionable substances. Outstanding figures in this work are Backhaus and his associates who, among other things, utilized activated carbon, another product of chemical ingenuity, to absorb the undesirable things from the gas. I shall not attempt to trace the history of the develop ment of carbonated beverages. A carbonated beverage is essentially a solution of carbon dioxide, sweetened, fla-
